1. Field of the Invention
The present invention relates to a thin-film capacitor element suitable for use particularly in a high frequency circuit among various electronic circuits.
2. Description of the Related Art
Electronic circuits are becoming increasingly smaller with the development of integrated circuit techniques. Accordingly, compact electronic circuit units have been proposed which have thin-film circuit elements, such as resistors, capacitors, and inductors, on an insulative substrate.
Among the circuit elements, known general thin-film capacitor elements (thin film capacitors) are formed by successively laying a lower electrode, a dielectric layer, and an upper electrode on an alumina substrate (for example, refer to Patent Document 1). The lower electrode is formed by sputtering chromium or copper on the alumina substrate and etching it into a desired pattern. The dielectric layer is formed by sputtering silicon dioxide or the like onto the lower electrode and etching it into a desired pattern. The upper electrode is formed by sputtering chromium or copper on the dielectric layer and etching it into a desired pattern. When the thin-film capacitor is used as a grounding capacitor, either one of the lower electrode and the upper electrode of the thin-film capacitor is routed to the outer rim of the alumina substrate and is connected to a ground electrode formed on the back of the alumina substrate through an end electrode.
[Patent Document 1]
Japanese Unexamined Published Patent Application No. 2001-339243 (pp. 5–6, FIG. 8)
The above-described known art requires a wiring pattern with a specified length to connect the thin-film capacitor formed on the surface of the alumina substrate to the ground electrode on the back. Therefore, the wiring pattern causes an inductance component to be increased, thus having the problem of increasing impedance loss when used in a high frequency circuit. Accordingly, a thin-film capacitor has been proposed in which either one of the lower electrode and the upper electrode is connected to the ground electrode through the use of a via hole in the alumina substrate.
FIG. 5 is a plan view of the known thin-film capacitor element; and FIG. 6 is a cross-sectional view taken along line VI—VI of FIG. 5. As shown in the drawings, a via hole 10a in an alumina substrate 10 is filled with a conductive material 11 such as silver paste and the lower end of the conductive material 11 is in conduction with a ground electrode 12 formed on the back of the alumina substrate 10. The thin-film capacitor element formed on the surface of the alumina substrate 10 has a multilayer structure of a lower electrode 13, a dielectric layer 14, and an upper electrode 15. The lower electrode 13 is in conduction with the top of the conductive material 11 in the position out of the multilayer structure.
In the thin-film capacitor element with such a structure, the lower electrode 13 connects to the ground electrode 12 through the conductive material 11 charged in the via hole 10a. Therefore, the inductance component can be reduced to a certain extent as compared with that with a long wiring pattern. However, the end face of the conductive material 11 made of silver paste or the like has seriously lower flatness than the surface of the alumina substrate 10. Therefore the part conducting between the lower electrode 13 and the conductive material 11 needs to be sufficiently separated from the part overlapping with the upper electrode 15 which specifies the capacitance value of the capacitor; the length of the lead section therebetween disadvantageously prevents further reduction in the inductance component.